The present invention relates to a method of treating the surface of a materiel or of an object by means of plasma generated by an electric discharge, and to a device for implementing the method.
A plasma generator is described in International Patent Application WO 97/18693. That generator makes it possible to obtain a jet of plasma in the form of a curtain. The parameters of that plasma, such as, for example, its temperature or its composition, are uniform along the curtain, in a direction perpendicular to the direction of the plasma flow.
By using that xe2x80x9ccurtainxe2x80x9d plasma jet technology, various types of surface treatment can be performed, by subjecting the surface under treatment to a substantially orthogonal spray of plasma onto the surface to be treated, so as, in particular, to perform cleaning, stripping, or sterilization of the surface, or else so as to coat it with a film.
Unfortunately, the low energy efficiency obtained when implementing that curtain plasma jet technology is not conducive to its development.
It has been observed that the quantity of energy from the plasma that is available for performing the surface treatment is small compared with the quantity of energy required for generating the plasma curtain. This is due to heat dissipation, but also to loss of activated particles from the plasma, caused by thermal convection, diffusion, and conduction phenomena in the vicinity of the plasma curtain. In addition, that type of plasma loses a large amount of its activity as it moves away from the jet source.
It has also be observed that generating such plasmas is accompanied by high emission of ultraviolet radiation, which can even give rise to photo-ionization of the material whose surface is being treated.
That phenomenon, probably due to the relatively high thickness of the layer of the plasma curtain, is a disadvantage in micro-electronics, which is a field in which surface irradiation with ultraviolet rays must be particularly well controlled, e.g. when certain treatment operations are performed on silicon wafers.
Finally, that curtain plasma jet technology is not suitable for performing the treatment by sweeping surfaces of large dimensions or surfaces that are not plane.
An object of the present invention is to make it possible, with good energy efficiency, to treat the surface of a material or of an object of any dimensions and/or of any shape by means of plasma having a small optical thickness, thereby emitting only a very small amount of ultraviolet rays, and conserving a very high activity in the surface treatment zone.
To this end, the invention provides a method of treating the surface of a material or of an object by means of plasma generated by an electric discharge, in which method said discharge is stabilized by confining said plasma in the form of at least one string, and the surface treatment is performed by putting the surface in contact with the plasma string along said string.
The method offers, in particular, the advantage of conserving a very high level of activity in the surface treatment zone because the reactive particles of the plasma are generated in the immediate vicinity of the surface under treatment.
Various variants of the method of the invention are defined in particular by the characteristics of the sub-claims, namely:
a variant wherein said surface to be treated and said plasma string are subjected to a relative sweeping movement, the direction of the sweeping movement being different from the direction of the axis of said string.
a variant wherein said discharge is stabilized by confining said plasma string within at least one channel constituted in part by the surface under treatment.
a variant wherein another portion of said channel is constituted by an essentially dielectric wall in the form of a trough.
a variant wherein the composition of said plasma is sustained by a flow of gas particles, the flow being inserted and removed in a same section plane of the plasma string, perpendicular to the direction of the string, so as to control the composition of the plasma locally while avoiding any longitudinal dissipation of the plasma.
a variant wherein the insertion and removal of said gas particles is distributed along the plasma string.
a variant wherein the plasma string is subjected to the action of a magnetic field angularly positioned in a direction different from the direction of the axis of the plasma string, so as to create an Ampxc3xa8re force that influences the position of the axis of the string as a function of the nature of the desired treatment.
a variant wherein said electric discharge is fed with DC or AC emitted between two electrodes, each of which is constituted by a plasma jet whose axis intersects the axis of the plasma string, the direction of each of the jets being different from the direction of the electric discharge.
a variant wherein said electric discharge is fed by an AC source and wherein said discharge is stabilized by means of an electrode disposed along the surface to be treated.
a variant wherein the discharge is fed by a pulsed current source, the surface treated discontinuously in strips or bands and a plurality of sweeping passes are performed in order to treat the entire surface.
The invention also provides a device for implementing the method, the device being characterized in that it includes at least two electrodes organized to emit an electric discharge, and means for stabilizing said electric discharge by confining the plasma in the form of at least one string, said device being organized to perform the surface treatment by bringing the surface into contact along the plasma string.
In a first embodiment of the device, the means for stabilizing the electric discharge and for confining the plasma in the form of at least one string may comprise at least one channel constituted in part by the surface under treatment, it being possible for another portion of said channel to be constituted by an essentially dielectric wall in the form of a trough.
Devices are already known that make it possible to stabilize an electric discharge within a channel. That type of device, which may be referred to as a xe2x80x9cdiaphragm-stabilized plasmatronxe2x80x9d, makes it possible to insert with strength that can be varied the column of an electric discharge into a series of diaphragms in alignment. A device of that type is described by W. Finkelnburg and H. Maecker in xe2x80x9cElectrische Bxc3x6gen und thermisches Plasmaxe2x80x9d, Handbuch der Physik, Ed. A Flxc3xcgge, Vol XXII, Gasentladungen II, Springer Verlag, Berlin, 1956, pp 254-444.
However, that type of device is used only to generate plasma jets and no applications of that jet technology propose or suggest treating some or all of the inside surfaces of the diaphragms, or causing the surface under treatment to take part in any way in stabilizing the electric discharge.
In the device of the invention, the material or the object whose surface is to be treated may be secured to a moving support. In which case, the surface of the support may also take part in stabilizing said plasma in the form of a string.
The channel for stabilizing the discharge and inside which the plasma string is confined may be constituted firstly by a wall in the form of a trough and secondly by the surface under treatment that covers said trough. By means of the essentially dielectric properties of the walls of the channel, the electric discharge current is forced to pass through it. The discharge is thus stabilized in said channel. The plasma generated by the electric discharge is contained in the channel and takes the form of a string. However, it does not fully match the outline of the section of the channel.
The section of the trough-shaped wall may be of any shape, curved or angular, and it may vary along the channel. Preferably, said section is circular in shape over the entire length of the channel.
Preferably, the ratio between the size of the section of trough-shaped channel and its length is less than 0.5. Preferably, the size of the section of the channel is constant along said channel.
The surface to be treated can be subjected to a relative sweeping movement, in a direction that is different from the direction of the axis of the plasma string, so that the entire surface to be treated is put in contact with the plasma.
The surface to be treated may be of any shape. It may be a plane surface such as the surface of a sheet, of a film, or of a solid plate, or a surface of the xe2x80x9ccorrugatedxe2x80x9d type, or even a complex surface of a three-dimensional object such as the surface of a bottle, or of a portion of bodywork. In which case, the plasma string is organized to adapt to match the shape of the surface to be treated. In particular, the channel containing the plasma string may be provided in a flexible material, thereby making it possible to cause its geometrical shape to vary on sweeping a surface to be treated that is of complex shape.
Optionally, by generating a magnetic field passing through the plasma string, and depending on the relative angular positioning of its Ampxc3xa8re force relative to the direction of the plasma string that is created by the DC electric discharge, it is possible, in particular, to confine the plasma string either against the wall of the trough, thereby increasing the stabilization of the electric discharge, or against the surface to be treated, thereby increasing the effectiveness of the treatment.
In another embodiment, the device of the invention includes at least one xe2x80x9csidexe2x80x9d electrode, encased in a dielectric body and disposed along the surface under treatment, so as to stabilize said electric discharge.